Thermal insulation is a widely used method of reducing undesirable heat gains or losses to a minimum. One extremely efficient method of providing thermal insulation is to use an evacuated enclosure such as disclosed in U.S. Pat. Nos. 4,546,798, and 3,680,631. However, such evacuated enclosures usually involve the use of walls of fragile glass, or heavy and expensive metals. Expensive vacuum pumps are necessary and the time required to pump the enclosure down to the required vacuum level can be excessive, in many applications. While such materials and costs can be justified in sophisticated applications such as chemical plants, oil gathering and the aerospace industry, etc., they are totally unacceptable in the requirements for the mass production of consumer goods.
For instance a non-limiting example is in the manufacture of domestic or "semi-industrial" refrigerators where, for economy of energy consumption, it is necessary to thermally insulate the cold storage space. This is presently accomplished by the use of sheets of foamed plastic material. Unfortunately the production of this foamed plastic makes use of chlorinated hydrocarbons whose widescale use is considered to be an ecological disaster and legislation is gradually being introduced to drastically reduce or eliminate their use.
In an attempt to provide an alternative insulating medium to foamed plastic it has been proposed to utilize plastic bags filled with a fibrous or powdered insulating medium and subsequently evacuated. However there have been found problems of gas permeation through the plastic bag causing loss of vacuum and hence thermal insulation. Creating the original vacuum is a lengthy process due to restricting conductances through pumping tubulations. Outgassing of the components during life again contributing to loss of vacuum is a problem. A getter device, to maintain the vacuum has been suggested but it must be heated, to cause it to sorb gases, at temperatures higher than the melting point of the plastics used.